Signal transmission system



L. G. ABRAHAM SIGNAL TRANSMISSION SYSTEM Oct. 24, 1944.

t Filed March 27, 1943 4 Sheets-Sheet .Enga :EL

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\ SIGNAL TRANSMISSION SYSTEM Filed March 2v. 194s 4 sheets-sheet 2 Nea. Res'.- 2a En H .ao 29 9,1499) ,291,

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VAVAVAVA /aa l I E /N VE N TOR BV/QABRAHAM ATTORNEY Oct. 24, 1944. L G. ABRAHAM 2,360,926

SIGNAL TRANSMISSION SYSTEM Filed March 2v, 194:5 4 sheets-sheet 3 NEG. REJ'-I F/G. 6 E m mj-L- la j NEG. Res,

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BVLG. ABRAHAM A T TORNE V Oct. 24, 1944.

|. G. ABRAHAM 2,360,926

SIGNAL TRANSMISSION SYSTEM Filed March 27, 1943 4 Sheets-Sheet fdl WMI m l maar] 3 :L -l :Il [wmf-WUI' E n E g uf-u [1w-'1% E l u. mf-fm1 stm-- V m EIS-- /Nl/ENTOR By .G. ABRAHAM A7' TORNEV Patented Oct. 24, 1944 andere i UNITED" STATES t, PATENT OFFICE l g I A' l fakseozsfff SIGNAL TRANSMISSION'SYSTEM Leonard, G. Abraham, Madison,N.'1J.,assignor to v Bell Telephone Laboratories, j Incorporated, New York, N. Y., a `corporationofNew York Application March 27, 1943, `Serial N0. 480,748'

' 1s claims. torna-415 1 This invention relates to an arrangement for controlling transmission loss in a signal transmission system embodying a plurality of individual transmission lines having their terminals connected'together for-establishing the lines in cirf cuit. It relates more particularlyy to such system in which a switchless arrangement including negative resistors embodied in certain line terminals serves to vary the effective transmission loss at individual line connections, vdepending l on whether individual terminals are connected to similar terminals, or different terminals.

Heretofore, negative resistors have been used in signal transmission systems for the purpose of introducing gain at one or more spaced points therealong. Each of such uses involves the in troduction of the negative resistors between line by the impede-need the une terminati to which terminals having substantially constant impedances. Also, heretofore, resistance pads have been used at the line terminals of certain signal transmission systems lto regulate the eiective amount of transmissionloss thereat depending on whether such line terminals were connected to line terminals of the same kind or of a different kind. The selection of the kind of line lterminals used with a particular line isy based on the transmission capabilities of such lines, among other considerations. .This usually involv'es the use of electromagnetic relays, control pads and associated apparatus for switching in and switching out the resistance pads, and is expensive from both initial cost and maintenance considerations. The present invention is concerned with 4a switchless arrangement for controlling the effective transmission loss at individual line connections in a signal transmission system as individual line terminals comprising such connections are switched -from similar line terminals to different lines terminals, and vice versa.

The present invention contemplates a switchless control of the effective transmission loss at individual line connections in a signal transmission system in which the impedance of one of two line terminals constituting'l the individual line connections is changed from time to time.

The main object of the invention is to provide a switchless control of thejeflective transmission loss at the connection of the terminal of one line as the line terminal to which the former is connected is changed in kind in. an electrical sense from time to time.

Another object -isto provide in a single line terminal a switchlessv device Whose eiective transmission loss at a given time is determined the former is connected 'atthe given time.

'A'fur'ther object is to provide a switchless variationv of the eiective transmission loss at the connection of 'a line terminal which may be switched between two or more other line terminalshaving diierent impedances. Anotherobject is to provide a switchless apparatus for changing automatically the effective transmission loss of individual` terminals of a toll telephone line as theflatter terminals are switched from similar toll terminals'to local line terminals, andviceversa.

'One type of twoewire toll telephone system includes,iamong other apparatuaVaJ plurality of` toll-oiiicesi a `local office associated. with each toll oce, a toll line connecting each pair oi toll oiiices, a local line'connectingeach toll oice and the 'lo'cal'oiiice associatedtherewith, and a cord 'circuit at each toll oiiice to connect the toll and local ofces incircuit. As the individual toll terminals in certain offices may be `connected thereatl to 'anotherttoll terminal or a local terminal; it is necessary fo'r'reasonably high `quality talking on the toll system to vary the eiectivey transmission loss atthe individual connections -of the toll Aand 'local toll terminals, depending on whether the'toll terminals are connected to other toll terminals or local'terrninals. -i

naccordance with a specic embodiment of the present-invention, resistance means provided withpreselected negative resistance characteristics are effectively interposed in individual toll terminals. When: two such toll terminals are 'connected' together in'one two-wire toll ovice, the 'negative'resistance means of the' individual toll terminals provides acertai'n amount of eiective transmission loss or 'gain at such connections. `When one of 'two such toll terminals 'of the one two-wire toll oice is connected to a local terminal in that toll office-the negative resistance means provides a diiierent amount of eiective transmission lo'ss `or 'fgain at such connection. The difference between such two losses or gains may be-made 4equivalent in losscliang'e to the mechanical insertion or removal of switching pads as mentioned above. l Y

In the caseof :four-wire' toll telephone systems, 'suchjneg'ative .resistance means is electivelyinterposgd ineach loi the transmitting and receivingv branches of individual Aone-Way paths. When two"such four-wire toll terminals are connected dividual connections in each direction of transmission. This has the effect of inserting a resistance pad of a preselected xed amount oi loss in the individual transmitting and receiving terminals of each one-Way path. When one of4 mission loss in individual connections in each,

direction of transmission, but such certain amounts of transmission loss are dilerent from each other as well as from amounts of ,transmission loss mentioned'above with -reference to f Fig. 1 shows a toll telephone system comprising a plurality of local central oinces I0, II and I2 connected over individual local transmission lines I3, I4 and I5 to individual toll telephone oces I 6, II and I8, respectively. Each local line terminates in the individual toll ofces in a telephone jack I9. A toll line connects toll oices I6 and kII and terminates in each thereof in a telephone jack; and a toll line 2I ljoins toll oiiices II and i8 and terminates in each thereof in a telephone jack. Toll offices I6 and I8 also embody further toll lines 20a and 2Ia, respectively. A subscribers 'telephone subset 22 is applied to each local oiice. The individual toll oftlces also the connection of two four-wire toll :terminale f A feature concerns the automatic change of effective transmission loss or *gain at individual line connections in accordance with the Arelative magnitudes of the impedances of the two Vline terminals constituting the individual line :con-

nections. Another :feature relates-to the eccomlplishmen'tpf .suchfohange :of loss or gain `without the aid of electromagnetic `switching apparatus.

. A further Aieature involves the 'use `of negativel resistors .zat individual line terminals to provide a switchless change ofloss or gain atindividual lline connections Ainvolving such terminals, depending on the :relative `magniinicles ofA the impedances of eachk ofthe `two line `terminals 'constituting .the :individual Vline connections. Another .feature includes the locationhof negative :resistors in series or shunt of certainline terminals and `the connection of .each of such `line terminals to .a vsimilar line .terminal or a -diierent iline terminal ,fforcontrolling .transmission loss or gain zat-such line connection.

.A further 'feature embodies `the smisrnatching of the v impedances :of two line vterminals connected together in circuit byfmeans of ,either `a transducer'alone or .a negative resistor alone v:for controlling-transmission loss'or gain at such line connection. .Another feature concerns the use vfor a transducer-ratione line terminaland negative `resistors at each of .two ,connected line terminals f including the sone line ,-zterminal for controlling transmission loss for gain at such connection. Still another lfeaturerelates'.to the use of transducers or negative resistors :.or both xateach 'of two utoll terminals for controlling transmission loss or gain Vat individual rtoll connections :com- -prisingsuch toll terminals fand local terminals,

vnwithout :the aid of .electromagnetic switching apparatus. Still Vanother feature involves the use Yof such transducers or inegativeresistors orfbotli individual terminals fof two-wirean d rfour.- 4wiretoll terminals.

The invention will be Vreadily :understood from `the-following description taken together .withthe Yaccompanving drawings in which:

sFig; :1 is a schematic circuit yshowing a ispecie A:erribodiment ofthe invention in connection l'with a two-iwiretoll telephone system;

Figs. .2V through 9A :illustrate modifications of the embodiments of `time invention shown Figs.

Fig. l0 is a schematic circuit of -a Vfour-wire ,toll .telephoneroiiice whichrmay embody the modiiicaticns tof the `invention :shown in Figs. 2

Athrough 99 :and which may be substitutediiiFig. :i

.to Aestablish a iourfwire ftoll atelephonezsystem.

In the following `description, the -same reference-numerals l.are-.em plot'ed to designate identi- .calelements appearing ,in the .several nguressof ,thel-.d-rawinga einoodye telephone cord circuit 23 for connect- Ling .the localand toll oiiices in circuit in the familiar manner so that talking toll circuits can beestablishedfbetween any two desired subscribers telephone subsets.

In accordance with a specific embodiment of the present invention, as .illustrated in Fig.. :1, a Vresistor .28 provided with -a l,preselected negative resistance characteristic, i for la purpose that will be herein-after mentioned, is Aeifectiveliv interposedinseries in the tolloice terminal of individual toll lines 2.0.2011, 2| and 24a in .the manner -shown in Fig. 2. Referring-talug. 2, a repeat `coil 29 has winding 2:9avconnected infseries in the yupper conductor of the toll line Yand coupled to split winding 29h vwhich is .interrupted substan- ,tially at its mid-point bya capacitor 30 and Vwhich has the negative l-resistor2ll connected in shunt thereof. A .source 3i Aof direct potential is dis- .pcsed in Ebridge of `the capacitor 30 for biasing :the negative resistor 28 rto Acontrol the point of ,operationen its characteristic. The repeat coil 2.9 also embodies .an identical pair of -coupled windings, .directcurrent source, and capacitor for where v.e is the-driving voltage.

Aeffectively connecting anothernegative resistor :Zinasimilar manner inseries in the lower conductor of the toll line terminalofFig. 2.

. In `the Aoperation of Figs, ,l and 42, let it besupposed Athat a subscriber at the local cnice `II) wishes .to call a .subscriber at the local office I2. To accomplish such call. the cord circuit of toll .olice II is inserted into the 'terminal jacks oi the local line I 3 `and the toll line 2.0; `the cord circuit ofjthe toll oi'lice I'I isfinsertedinto the terminal jacks of the toll lines '2U and 2l; and the cord circuit of the toll oiiice I8 .is inserted into Vthe Vvterminal jacks of the toll line '2| and the local line I5. This establishes a circuit for talking `between the subscribers located at the local oices Ill-and I2. 'v

*In order that the 'amplitudes of the talking currents ytransmitted between local stations I0 vand l2 :beef such orderof magnitude as to enable intelligible talking therebetween, the gain or transmission'loss of the previously traced talking .circuit should'be iixed at a certain order of magnitude, consistent with the over-al1 length of the ,effective talking circuit. This may be achieved -initheffollowing manner: Vl'teferring to Fig. 1, it is seen that the previously traced talking circuit includeslocal lines I3 and I5, land toll ylines 20 and 2I. Let Lbe the impedance of each .of the "local and toll lines. When two lines of impedance :If -a negative resistance -r is effectively inserted in series in one'of the two :lines of imped- 2,360,926 ance L connected in circuit, the current I1 now-v` ing therein is Y f Ver s 2? The gain inserted in the circuit of Equation2 with reference to that of Equation 1 is If a negative resistance v-r iseiectively connected in series in each of the two lines of impedance L, the current I2 flowing therein is e I2: z ll 2T l l(4) The gain inserted in the circuit of `Equation 4 with reference to that of Equation 1 is For convenience we may write If now we select T v I then i G1=20 log (1- m =4.67 decibels (6) G2=20 log 15.3 decibels [h-1.65% decibels] from each of the two lines L connectedtogether and having a negative resistance embodiedin series therewith. Obviously, further increases of gain can be realized by connecting in circuit `additional lines of impedance L and yequipping'each of the latter with negative resistors '-r of the series type indicated in Fig. 2.

As the talking circuit described above with reference to Figs. 1 and 2v includes a local line terminal having no negative resistors and a, toll line terminal embodying a negative resistor effectively in series therein connected together at individual toll ofces I6 and I8, it is apparent from Equation 6 that the negative resistor introduces a gain of 4.65 decibels in each ysuch line connection. Also, as this talking circuit further includes two toll line terminals, each embodying a negative resistor in series therein, connected -together at toll office I'I, it is apparent from Equation 'I that the negative resistors cooperate. to 1 introduce a gain of 15.3 decibels in such connection. As previously pointed out, the latter gain is equivalent to the disconnection of a 3 decibel resistance pad from each of the two toll terminals. In Fig. 2, repeatcoil 29, dueto the adjustment of the impedances at its terminalsypermits' the use 4of varying sizes of negative resistors for introducing a` given `amount of gain or loss in individual toll line connections. 'Theuse of this repeatmay be advantageous when thesizes of the available negative resistors are limited.

Now, let it be supposed that a subscriber at local oilice IIJ Wishes to call a subscriber at local oice II. To accomplish this, the cord circuit vof toll oiiice I6 is inserted into terminal jacks of localand toll lines I3 and 20, respectively, and cord circuit of vtoll oiice I'I is inserted into the terminal jacks of local and toll lines I4 and 20, respectively. Each toll line terminal is equipped with series negative resistors according tof Fig. 2. As each toll line terminal embodying a negative resistor in series therein is connected, atthe toll oices I6 and I1, to a local line terminal embodying no negative resistors, it is apparent from Equation 6 that the negative resistors at the individual toll terminals introduce a gain of 4.65 decibels in eachsuch line connection.

Fig. 3 is similar to Fig. 2 except negative resistor 28 is effectively'interposed in shunt of the toll line terminal. For'this purposarepeat coil 33 has its respective primary and secondary windings 33a and 33h applied across the toll line'terminal, and a split tertiary winding 34 coupled to both of these windings. Interposed at the midpoint of the tertiaryr winding 3,4 is ay capacitor 35 in bridge of'which is a source'36 of a direct current for so biasing the negative resistorv 28 as to control the point of operation on itscharacteristic. It can be demonstrated in connection with Figs. 1 and 3 included in thepreviously described talking circuits involving two 'local offices in circuit with two or `three toll oiiices that (l) when a local line terminal having no negative resistor is connected to a toll line terminal embodying a negative resistor in shunt thereof in accordance with Fig. 3, the negative resistor introduces a certain amountof gain into each line' connection so equipped, which amount of gain` is different from that introduced by series negative resistors according to Fig. 2.; and when two toll terminals embodying negative 'resistors in shunt thereof in accordance with Fig. 3 are connected together, the ynegative resistors introduce a gain of 15.3 decibels into such connection, which gain is identical with that introduced by series negative resistors according to Fig. 2. This gain' is equivalent to the disconnection of a 3 decibel'resistance pad from each of the two toll terminals.

s However, when a toll terminal embodying a negative resistor of the series` type, Fig. 2, isconnected to a toll terminal embodying` anegative resistor of the shunt type, Fig. 3, a further'amount of transmission loss orv gain is inserted in suchcon'- nection, which further amount is diiTerent from the certain amount mentioned above with reference to the connection of two terminalsembodying negative resistors of the series type, Fig. 2,

or two terminals embodying negative resistors of the shunt type, Fig. 3. f

By preselecting the characteristics of the individual negative resistors, each line connection embodying a toll terminal can be provided with a predetermined amount of transmission loss or gain whereby the amplitudes of the talking currents in' a given toll circuit can be so controlled as to improve the. quality of talking transmission therein. Such improvement is, of course, not limited to the toll circuits identified hereinabove, but can be obtained in a tollV circuit embodying a number of discrete toll lines invexcessfof three.

Inthe foregoing toll circuitaitis understocd'that the net transmissionloss or :gainof Athe entire toll circuits may'be adjusted-by means, other than those, mentioned herein, to establish the 'proper level of ltransmission between :the twolocal .sub-v scribers for obtainingr`1 reasonably high .,quality talking'iin theA toll circuit extending therebetween.y

Thus, lwhen aline "terminal equipped'with a negative resistor of Vthe series or shunt type, Fig. 2.01' 3, is connected to a line terminal `unequipped with `a negative :'resistor, the series negative-resistor. introduces one amount1 of gain or lossvin such xconnectionfwhileth'e shunt negative resistor introduces a diiferentgamount of gain or iloss in such connection. "When two :line-terminals, each equipped with a -vnegative resistor-'of the :series type, Fig.' 2, are connected togethena rcertain amount of transmissionloss orgain is introduced A mission loss or gain mentioned above `with refer*v ence to the other types of terminal connections.

Figf4 islzsimilarvto Fig. 2 except a repeat coil 31 is:interposed'ibetweenthe negative resistors and the toll terminal jack. Repeat coil 31 varies the eiective lvalue of the terminating impedance of the toll line as seen from the negativeresistors. As. the. effective rtransmission loss or' gain established inaV line connection embodying the terminal ofFig. 4,is.determined by the impedance of the terminalv to which the'fnegative resistor terminal, Fig. 4, is connected in such line connection, the repeat coil 31 serves to supplement the effectiveness of Vthe negative` resistors for varying the transmission loss or gainin such connection, provided such connection embodies only one repeat coil 31.`

*Fig 5 is similar to Fig. 3, except it alsoA embodies repeat Acoil V31 interposed between the nega'- tive resistor andthetoll terminal jack. Repeat coil 31 controls the eifective Avalue of the terminating impedance ofthe vrespective toll line as seen from the negative Aresistor and thereby the effective'transmission loss or gain in a line connection embodying one repeat coil31 only according to Fig. 3 aspreviously pointed out in connection'with aV lineconnection embodying one repeat coil 31 only according'to Fig. 4.

In certaincases'the negative resistors, Figs. 4

-and 5, may be omitted so that the eiective transmission loss or gain at a line connection embodyingfone repeat coil 31 only is controlled by the impedance effects of the repeat coil 31 alone, Fig. 9A. When two toll terminals, both of which are equipped 'according'to Figs.4 and 5, are connected togetherythe impedance eiect due to the repeat coil 31 of one toll terminal is cancelled by the impedance effect due to the repeat coil 31 of the other terminal, and hence the two vtoll terminals, Fig. 4 or 5, simplify essentially'to .two toll terminals `according to Fig. 2. Hence, when two line terminals equipped according to Fig. 9A are connected together, the netloss at such connection is substantially zero; but when one ter-l minal is equipped according toFig. 9A andis con-V accesso:

nected to another-ternnal unequipped withthe repeating coil 31 of Fig. 9A a certain magnitude of reflection loss is eiiectively introduced in such connection. `When vtoll terminals according to Fig. 4 or Fig. 5 are individually connected to a local line terminal, the individual toll terminals introduce individual amounts of transmission loss or gain in the connection such that the amount of loss orgain due to the toll terminal of Fig, 4 is differentl from that due to the toll terminal of Fig. 5. This is equivalentv to the insertion or removal of individual resistance pads whose effective resistance values correspond in magnitude to the impedances of the negative resistor terminals according to Figs. 4 and 5.

Figs. 6 and 7 are similar to Figs. 4 vand 5, respectively, except impedance networks 38 are substituted for the repeat coils 31 for controlling the terminating impedance' of the individual toll terminals. Networks 38 may comprise resistance pads, autotransforniers, and similar transducers.

Figs. 8 and 9 are similar to Figs. 6 and '1, respectively, except impedance networks 38 are disposed on the linefsides of the negative resistors. Fig. 8 is similar to Fig. 6 except identical impedance terminating networks 38, 38 are disposed on opposite sides of the negative resistors 28, 28. Fig. 9 is similar to Fig. '1 except identical impedance terminating networks 38, 38 are disposed on opposite sides of the negative resistor 28.

Fig. 1U shows a four-wire toll oice which comprises two one-way paths and which may be substituted for the individual two-wire toll ofces I6, I1 and I8, Fig. 1. For example, the toll oiiice of Fig. 10 may be substituted for the individual toll oiiiceslIG, Hand I8, Fig. l, by connecting the local lines to the proper local terminal and toll lines to the proper toll terminals. ferring to Fig. 10, seriesnegative resistors of any one of the types shown in Figs. 2, 4, 6 and 8 may be effectively interposed in the receiving branches of each ofthe vand E -W one-way paths while shunt negative resistors of any one ofthe types shown in Figs. 3, 5, 7 and 9 may be inter# posed in the transmitting branches of each of the W--E and E-W one-way paths. In one representative case, when a toll terminal was connect-l ed to a local terminal there was obtained a 24.6 decibel receiving gain and a 2.5 decibel transmitting gain in each of Ythe one-way paths; and when the two-way toll terminals were connected together, an 11.1 decibel gain was obtained in the individual 'transmitting andreceiving branches of each one-way path. The 11.1 decibel gain is substantially the equivalent of inserting an 8 decibel resistance pad,

in the individual transmitting and receiving terminals of each one-way path when two four- Wire toll'terminals are connected together. This effect is opposite to that described above for the operation of Figs. `1through 9 where the effect, when identical two two-way toll terminals were connectedtcgether, was thata 3 decibel resist'- ance pad was effectively removed from each of the toll terminals. It is to .be understood that the same result could be obtained when the series and shunt negative resistors are effectively connected in the individual transmitting and receiving branches, respectively, or in various other combinations.

What is claimed is:

'1. In combination.. a plurality of lines for transmitting signal current, means to Aconnect the terminals `of said lines for arranging said lines in circuit, and means comprising networks having predetermined impedance versus frequency characteristics and disposed in the terminals of certain of said lines for automatically controlling the eiective impedance to said signal current when a line equipped with said network means is connected in circuit, said network means being so constructed and arranged that the effective impedance at a connection of the terminals of two of said certain lines is less than the impedance at the connection of a` terminal of one of said certain lines with the terminal of another line not .equipped with said network means.

. 2. In a signaling system, a switching point, a plurality of lines for transmitting signal current terminating thereat, certain of said lines having a first terminal impedance characteristic and the remainder of said lines having a second characteristic, switching means for interconnecting the terminals of'desired ones of said lines, and individual static means associated with the terminals of the lines ofthe first characteristic so constructed and arranged that the effective impedance to said signal current at the connected terminals of two lines of said rst characteristic is less than the effective impedance at the connected terminals of two lines only one. of which -is of said first characteristic.

3. The combination according to claim 1 in which said networks include resistance means provided with `preselected negative resistance characteristics.

4. rI'hecombination according to claim 1 in Vwhich said networks include resistance means provided with preselected negative resistance characteristics, and electrical transducers positioned in preselected certain terminals.

5. In combination, a plurality of lines transmitting signal current and having predetermined magnitudes of terminal impedances for the signal current, means to connect the terminals of ysaid lines for arranging said lines in circuit, and means to control the effective impedance for the signal current at individual connections of the terminals of certain lines,` including means pro'- `vided with preselected negative resistance characteristics and interposed in each of said certain line terminals to establish an effective impedance of a lesser degree of magniture at'the connection of two of said certain terminals than at the connection of oneof said certain terminals .with a terminal rnot providedl with means including a negative resistance.

6. In combination, a plurality of first and second type lines for transmitting signal current and having predetermined terminal impedances to said signal current, means to connect desired terminals of said lines to establish a transmission circuit thereover, and static impedance reducing means, including a network comprising a predetermined negative resistance, disposed in each of the terminals of said first type lines so arranged as to reduce to a lower value the overall terminal irnpedance when two of said first type lines are connected than when first and second type lines are connected.

7. In combination, a plurality of first and second type lines for transmitting signal current and having predetermined magnitudes of terminal impedance to said signal current, means to connect desired terminals of said lines to establish a signal transmission circuit thereover, and

l impedance reducing means including a negative `ing office and a plurality of first and second type :resistance of preselected value interposed in the terminals of said first type lines to-reduce the effective impedance to signal 'current applied thereto, said negative resistance means having the' characteristic of reducing the over-all terminal impedance at two connected lines of the first type to a lower value than the terminal irnpedance at the connection of a first type line with a second type line. K

8. In a telephone system comprising a switchsignal `current transmission lines terminating thereat in impedance having a predetermined vvalue when said signal current is transmitted thereover, switching means at said oice for connecting the terminals of any two of said lines together, and impedance reducngmeansincluding Ia negative resistance associated with the terminals of said rst type lines and so constructed and arranged that the effective impedance to said signal current at the connection of two of such first type lines is reduced to a lower value thanl the impedance reduction at the connection of a line of the first type with one of thel second type.

9. The telephone 'system according to claim 8 vin which said negative resistance means is effectively connectedin series to each of said cer, .tainzline terminals.

10. T he telephone system according to claim 8 vin which said negativev resistance means is effectively connected in shunt of each of saidcer- `tain line terminals.

11. The telephone system according to claim 8 infwhich the said negative resistance associated with at least one of said lines is connected in effective series relationship therewith and the negative resistance associated with at least one other of said lines is connected in effective shunt relationship therewith and at least one of said terminal interconnections involves one terminal having an vassociated series type negative reshunt type negative resistance.

y 12. A toll telephone system comprising aplurality of toll o iices, a local central oiiice assosistance and one terminal having an associated ciated with individual toll offices, a line for transy mitting signal current between each local oiiice and one toll office and terminating in said latter oflice in `one magnitude of impedance for the .signal current, a line for transmitting the signal .current between associated pairs of toll officesY and terminating in said latter oflices in a different magnitude of impedance for the signal current, means at individual-toll oces to connect the terminals of said, lines 'for establishing at least three toll offices and two local offices in circuit, and means including means provided -with a preselected negative resistance characterminal is connected to one local line terminal f at another toll omce in said circuit said negative resistance means of said one toll line terminal varies the magnitude of the effective m- `pedance thereat to establish a different amount offefective transmission loss at the'connection other toll oflice. l

of said one toll and local line terminals at said 13. A toll telephone system comprising a plurality of toll oices, a locall central cniceassociated with each of said toll oiiices` atoll line extending between each pair of toll ofces, a local line extending between each toll oce and the local oflice associated therewith, means at each toll omceto connectfat least three toll oii'ices and at leastv two local oices incircuit, and means including means provided with preselected negative resistance characteristics and interposed in individual toll line terminals at each of said toll otces to Vcontrol' the effective transmission loss at individual toll cnice connections which include -toll terminals connected to toll terminals and toll terminals connected to local terminals. Y

Y 14. A telephone system comprising a switching oflce, a yplurality of transmission lines each of Whichis terminated in four wires in said ofce such that each two -wires serves -to transmit` in opposite directions, means to connect two of said lines in circuit at said cnice, and means includving means provided 'with a preselected negative lresistance characteristic andconnected eiectively in each two-wire terminal of certain fourvwire lines to control the effective transmission loss in individualtwo-wire terminals of said certain four-wire lines such that when two'certain four-wire lines are connected together in said circuit said negative resistance means individual to eachof the-two-wire terminals transmitting i'n the. same direction establish substantially equal amounts of effective transmission loss in each of said `latter two-wire terminals and such that when one certain four-wire line is connected to one other four-wire line" in saidcircuitsaid negative resistance means individual to the twowire terminals ofsaid one certain four-wire line established other vamounts ofeiective transmission loss which other amounts are different from each other and different from said equal amounts of transmission loss.`

- 15. The telephone system according to claim 14 in whichone of said negative resistance means is connected eiectively in series one two-'wire terminalfof each of said certain four-wire lines, and another of-said resistance means is -connected effectively in shunt of the other two-wire terminal of each of said certain four-wire lines.

16. The telephone systeme-according to claim 14 in which individual certain four-wire lines comprise a Vreceiving two-wire terminal and a transmitting vtwo-wire terminal for transmitting fascacac in the same direction-,viand'one of saidl negative resistance. means is connected effectively in series in the individual receiving vtwo-wire terminals, and another of said resistance means is connected effectively in shunt of individual transmitting two-wire terminals.

Y 17. A toll telephone system comprising4 a'plurality of toll oices, a. local central oce. associated with each of said tollv offices; a four-wire toll line extending between each `pair ot toll offices, a locaL line extending between each local office and the. toll oiice associated therewith and terminating in the latter oice in a 'four-wire line, means at said toll omces to connect at least three toll oices and two local offices in circuit. and means including means provided with preselected negative resistance.characteristics and interposed'in individualtWo-wire terminals of each of vsaid four-wire toll terminals to control theeiective transmission loss at individual twowire connections of each of said four-wire toll terminals',` which connections. include toll terminals connected to toll terminals and toll terminals connected to local terminals.

18. A toll telephone system comprising. a plurality of toll offices,` a local central. cnice associated with each of said toll offices, a two-wire transmission vline extending between each toll and local cnicey and terminating in each toll cnice in a four-wire line, a four-Wire line extending between each pair of toll oices, means at said toll ofiices to connect at. least three toll offices and two local olces in circuit, and means including means provided with a preselected negative resistance characteristic and interposed in each two-wire terminal of individualrfourwire toll lines to control the effective transmission loss in said circuit such that when two fourwire toll lines are connected together in one toll office in said circuit said negative resistance means of the individual two-wire toll terminals off said four-wire toll lines at said one toll oli-ice establish substantially equal amounts of transmission loss in the respective individual two-wire toll'connections and 'such that when one fourwire'toll terminals of said one four-wire toll line -at saidother toll olice establish other amounts of. transmission loss inthe respective individual two-wire toll, local connections, which other .amounts are different from each other and different from said equal amounts of transmission loss. l

anormal) G.- ABRAHAM. 

